Estimate the required generator wattage for your home's essential appliances.
Your Home's Needs
Estimated Generator Size Needed:
— Watts
This is an estimate. For critical loads or more complex systems, consult a professional.
Understanding Generator Sizing for Your Home
When the power goes out, a home standby generator can be a lifesaver, keeping your essential appliances running smoothly. However, choosing the right size generator is crucial. Too small, and it won't power what you need; too large, and you've overspent unnecessarily. This calculator helps you estimate the total wattage required based on the starting (surge) and running (continuous) watts of your essential appliances.
How Generator Sizing Works
Most electrical appliances, especially those with motors (like refrigerators, pumps, and fans), require more power to start up than they do to run continuously. This initial surge in power demand is called "starting watts" or "inrush watts." The power needed to keep the appliance running after it has started is called "running watts" or "continuous watts."
A generator must be able to supply enough power to meet the demands of all appliances that might be running simultaneously. The critical factor is often the appliance with the highest starting wattage, as the generator must be able to handle that initial surge, plus the running watts of all other appliances that are on at the same time.
The Calculation Logic
The Generac Generator Sizing Calculator uses the following logic:
Identify Critical Loads: First, list all the appliances and electrical devices you absolutely need to power during an outage. This might include your refrigerator, well pump, sump pump, furnace fan, essential lighting, and critical electronics.
Find Starting and Running Watts: For each appliance, determine its starting watts (surge) and running watts (continuous). These values can often be found on the appliance's nameplate, in the owner's manual, or by searching the manufacturer's specifications online. If an appliance has a significantly higher starting wattage than running wattage, use both values.
Calculate Total Starting Watts: The most critical value for generator sizing is the highest single starting wattage among all your appliances, plus the sum of the running watts of all other appliances that could be operating at the same time.
Formula Simplified:Total Required Watts = (Highest Starting Wattage Appliance) + (Sum of Running Watts of ALL other appliances)
The Calculator's Approach: This calculator sums up the running watts of all appliances. Then, it identifies the largest difference between starting and running watts for any single appliance and adds that difference to the total running watts. This effectively calculates:
Total Running Watts + (Highest Starting Watts – Highest Running Watts)
This provides a robust estimate of the peak wattage your generator needs to supply.
Example Calculation:
Let's say you want to power the following essential items:
Step 2: Highest Starting Wattage Appliance
The Well Pump has the highest starting wattage at 1500 Watts.
Step 3: Calculate Peak Demand (using the calculator's method for simplicity)
We take the highest starting wattage appliance (Well Pump: 1500W) and add the running watts of all other appliances.
1500 (Well Pump Start) + 200 (Fridge Run) + 500 (Furnace Run) + 50 (Lights Run) + 150 (Computer Run) = 2400 Watts.
Alternatively, using the formula the calculator implements (Highest Starting Watts + Sum of Running Watts of *other* appliances):
Let's assume the Well Pump (1500 starting, 500 running) is the highest surge item.
Total Running Watts = 200 + 500 + 500 + 50 + 150 = 1400W.
Highest Starting Wattage = 1500W.
Highest Running Wattage = 500W.
Generator Size = Highest Starting Watts + (Total Running Watts – Highest Running Wattage)
Generator Size = 1500W + (1400W – 500W) = 1500W + 900W = 2400W.
Therefore, a generator rated for at least 2400 watts would be recommended for this specific load combination. It's always advisable to select a generator with some buffer capacity.
Important Considerations:
Add a Buffer: It's wise to choose a generator with 10-20% more wattage than your calculated need to ensure longevity and handle unexpected surges.
Appliance Variations: Wattage can vary significantly between models and even for the same appliance. Always check the specific ratings for your devices.
Whole-House vs. Essential Circuits: This calculator focuses on essential circuits. Whole-house generators are much larger and more complex.
Professional Consultation: For precise sizing, especially for larger homes or complex electrical systems, consult with a qualified electrician or a Generac dealer.
function calculateGeneratorSize() {
var startingWattsRefrigerator = parseFloat(document.getElementById("startingWattsRefrigerator").value);
var runningWattsRefrigerator = parseFloat(document.getElementById("runningWattsRefrigerator").value);
var startingWattsWellPump = parseFloat(document.getElementById("startingWattsWellPump").value);
var runningWattsWellPump = parseFloat(document.getElementById("runningWattsWellPump").value);
var startingWattsSumpPump = parseFloat(document.getElementById("startingWattsSumpPump").value);
var runningWattsSumpPump = parseFloat(document.getElementById("runningWattsSumpPump").value);
var startingWattsFurnaceFan = parseFloat(document.getElementById("startingWattsFurnaceFan").value);
var runningWattsFurnaceFan = parseFloat(document.getElementById("runningWattsFurnaceFan").value);
var startingWattsLights = parseFloat(document.getElementById("startingWattsLights").value);
var runningWattsLights = parseFloat(document.getElementById("runningWattsLights").value);
var startingWattsMicrowave = parseFloat(document.getElementById("startingWattsMicrowave").value);
var runningWattsMicrowave = parseFloat(document.getElementById("runningWattsMicrowave").value);
var startingWattsComputer = parseFloat(document.getElementById("startingWattsComputer").value);
var runningWattsComputer = parseFloat(document.getElementById("runningWattsComputer").value);
var startingWattsTV = parseFloat(document.getElementById("startingWattsTV").value);
var runningWattsTV = parseFloat(document.getElementById("runningWattsTV").value);
var startingWattsSpaceHeater = parseFloat(document.getElementById("startingWattsSpaceHeater").value);
var runningWattsSpaceHeater = parseFloat(document.getElementById("runningWattsSpaceHeater").value);
var startingWattsOther1 = parseFloat(document.getElementById("startingWattsOther1").value);
var runningWattsOther1 = parseFloat(document.getElementById("runningWattsOther1").value);
var startingWattsOther2 = parseFloat(document.getElementById("startingWattsOther2").value);
var runningWattsOther2 = parseFloat(document.getElementById("runningWattsOther2").value);
var appliances = [
{ name: "Refrigerator", start: startingWattsRefrigerator, run: runningWattsRefrigerator },
{ name: "Well Pump", start: startingWattsWellPump, run: runningWattsWellPump },
{ name: "Sump Pump", start: startingWattsSumpPump, run: runningWattsSumpPump },
{ name: "Furnace Fan", start: startingWattsFurnaceFan, run: runningWattsFurnaceFan },
{ name: "Lights", start: startingWattsLights, run: runningWattsLights },
{ name: "Microwave", start: startingWattsMicrowave, run: runningWattsMicrowave },
{ name: "Computer", start: startingWattsComputer, run: runningWattsComputer },
{ name: "Television", start: startingWattsTV, run: runningWattsTV },
{ name: "Space Heater", start: startingWattsSpaceHeater, run: runningWattsSpaceHeater },
{ name: "Other 1", start: startingWattsOther1, run: runningWattsOther1 },
{ name: "Other 2", start: startingWattsOther2, run: runningWattsOther2 }
];
var totalRunningWatts = 0;
var highestStartingWatts = 0;
var applianceWithHighestStart = "";
for (var i = 0; i < appliances.length; i++) {
var appliance = appliances[i];
// Validate input for running watts
if (isNaN(appliance.run) || appliance.run < 0) {
appliance.run = 0; // Default to 0 if invalid
}
totalRunningWatts += appliance.run;
// Validate input for starting watts
if (isNaN(appliance.start) || appliance.start highestStartingWatts) {
highestStartingWatts = appliance.start;
applianceWithHighestStart = appliance.name;
}
}
// The core calculation: Highest starting watts + sum of running watts of *all other* appliances.
// This is equivalent to: Highest Starting Watts + (Total Running Watts – Running Watts of the highest starting appliance)
var highestRunningWattsOfApplianceWithHighestStart = 0;
for (var i = 0; i < appliances.length; i++) {
if (appliances[i].name === applianceWithHighestStart) {
highestRunningWattsOfApplianceWithHighestStart = appliances[i].run;
break;
}
}
var requiredWattage = highestStartingWatts + (totalRunningWatts – highestRunningWattsOfApplianceWithHighestStart);
// Ensure a minimum if all inputs are zero or negative, though our logic should prevent this
if (requiredWattage <= 0) {
requiredWattage = 500; // A very basic minimum for any scenario
}
document.getElementById("result-value").innerText = requiredWattage.toLocaleString() + " Watts";
}